Oil-based mud (OBM) contamination monitoring (OCM) was developed to estimate the contamination level of incoming fluid during non-focused sampling. For focused sampling, the same OCM approach has been used to estimate the contamination of synthetic commingled fluids utilizing measured flow rates of sample and guard flowlines, assuming that the commingled flow of a focused sampling tool behaves like a non-focused sampling tool. However, in focused sampling, the contamination level in the sample flowline cannot be accurately estimated during early phases of cleanup because it is too early to accurately estimate the optical density (OD) of a formation fluid using the behavior of a commingled flow. That is, average contamination is still too high to obtain accurate estimation, due to slow cleanup at the guard inlet. Moreover, the commingled behavior of a focused sampling tool is not identical to the cleanup behavior of a non-focused sampling tool, such that large discrepancies are observed with non-zero differential pressure between the sample and guard inlets. The computation of commingled flow based on flow measurements with error may also introduce greater uncertainty.